This invention relates to an x-ray-based method and apparatus for measuring thickness and/or composition of samples; more particularity, the invention relates to the use of coherent: x-rays generated using short, optical pulses to measure properties of thin films contained in multi-layer samples.
Thin films of dielectric (e.g., polymer, oxide) and conducting (e.g., metal) materials are used in a range of microelectronic, optical, and biomedical devices. A microprocessor, for example, contains multiple layers of metal and oxide thin films deposited on a semiconducting material (e.g., a silicon substrate). Thickness and compositional variations in these films can modify their electrical and mechanical properties, thereby affecting the performance of the microprocessor. Accordingly, film thickness and composition are often monitored as quality-control parameters during and/or after the microprocessor""s fabrication.
Several film measurement techniques, such as optical ellipsometry, reflectometry, Impulsive Stimulated Thermal Scattering (ISTS), and x-ray fluorescence (XRF) are known to measure oxide and metal films during fabrication of a microprocessor. In XRF, a beam of x-rays is generated and collimated using conventional x-ray optics; in this case, the beam that irradiates the sample typically has a diameter on the order of 1 cm.
Alternatively, the collimated x-ray beam can be passed through a mask that spatially filters it to generate a beam having a diameter of a few hundred microns. The disadvantage of this technique, of course, is that the filtered x-ray beam has a significantly lower power than the x-ray beam before the mask.
Techniques to generate X-rays using laser-based methods are also known. In one such technique, a high-peak-power femtosecond laser pulse is focused on an atomic gas. The interaction of the laser light and atomic gas creates emissions of coherent high-order harmonics. This essentially changes the femtosecond pulse""s energy from the visible spectrum region to the soft-x-ray spectrum region. In addition, X-ray laser sources are also know in that produce coherent radiation.
The inventor has discovered significant advantages in the use of coherent x-rays in performing measurements as compared to conventional measurement and x-ray fluorescence techniques. These benefits and advantages overcome numerous shortcomings of conventional XRF, as discussed below.
In the present invention, the use of coherent x-ray emissions provides improvements over conventional measurement and x-ray fluorescence techniques. Coherence refers to the correlation between propagation characteristics of a signal at points separated in space, or in time, or both.
The advantages of the present invention include the use of high intensity coherent x-rays. As discussed above, the spatial filter, i.e., the mask, used in conventional XRF significantly limits the x-ray power output. Moreover, even with such filtering, the XRF beam typically has a diameter on the order of 1 cm. In the present invention, the coherent x-rays allow for extremely small spot sizes to be measured (e.g., on the order of a few microns or even well under 1 micron) without the need for any spatial filtering.
In addition, the spatial coherence of such x-rays provides advantages in measuring thickness and compositions. Such coherent x-rays, for example, allow measurements to be made at longer distances to sample than conventional XRF. In measurements in which beams are crossed (e.g., ISTS measurements), coherent x-rays allow for greater spatial resolution. Depending on the focussing optics, either the spot sizes or fringe spacing resulting from the crossed x-ray beams can be in the submicron range.
Also, the short pulse duration of such coherent radiation allows for time-resolved measurements, or measurements as a function of time delay between two pulses, e.g., an x-ray pulse and an optical pulse or two x-ray pulses. For example, one pulse excites electrons, and another pulse (preferably a different wavelength) is absorbed by the excited states to produce a still higher-lying state that fluoresces. It is noted that the delay between the pulses is important because the initially excited levels are short-lived. Combined x-ray and visible excitation pulses may also be useful in this respect. For example, the x-rays excite core electrons, and then the visible pulses (readily available and synchronizable since these visible pulses are also used to generate the coherent x-rays) are used to make measurements (time-resolved or otherwise) on the excited electrons or on the xe2x80x9cholesxe2x80x9d left behind (the latter includes optical or x-ray transitions that promote electrons from a filled core level into the core level that was vacated by the first x-ray pulse). Such measurements add selectivity for particular species, since now more than one transition frequency and also a decay lifetime can be matched.
Another advantage discovered by the inventor is based on the simultaneous availability of multiple x-ray wavelengths. For example, different x-ray harmonics may be selected from two supply fibers (as discussed below) or from a single fiber. Typically a single fiber output may consist of a series of nearby harmonics (e.g. harmonics #100, 101, 102, 103, 104, 105 . . . ). This allows for a xe2x80x9ccombxe2x80x9d of evenly spaced frequencies and nearly evenly spaced wavelengths. Having various wavelengths available in this form enables several useful measurements, as follows:
(1) Comparison between XRF emission generated from two or more wavelengths may be used as a basis for discrimination between different materials. In general, an x-ray xe2x80x9cexcitation spectrumxe2x80x9d (intensity of fluorescence at a fixed wavelength measured as a function of excitation wavelength) may be collected.
(2) Soft x-ray interferometry/reflectometry. This is useful for ultrathin films and for films that are opaque to visible light. In this regard, in spectroscopic and variable-angle interferometry multiple wavelengths and angles for coherent high input are used. As the angle of incidence is varied, the optical path length reaches an integer multiple of each wavelength in succession, i.e., it depends on the peculiarity of the evenly spaced harmonic wavelength spectrum.
(3) Correlation measurements involving different wavelengths. For example, first assuming that wavelength 1 promotes core electrons from material 1, this leads to fluorescence when those electrons return to their core levels. Also, assume that wavelength 2 does the same for material 2. The question is whether wavelength 2 increases fluorescence at wavelength 1 when both are applied. If the materials 1 and 2 are well mixed, this is the case. If the materials 1 and 2 are in immiscible clusters or domains whose sizes exceed the electron diffusion length, this is not the case. This embodiment is useful for solid-state alloys or concrete, for example.
Another question is whether there is a time delay when performing such correlation measurements. In this embodiment, measurement of the average separation of the materials can be made (e.g., adhesion and delamination properties can be measured). This separation can either be due to incomplete mixing or because the materials 1 and 2 are in a multilayer assembly and there""s a film or an interface between them (e.g., an electron transport across the materials).
In view of these discoveries, it is an object of the invention to provide a method and apparatus for measuring a property of thin films using coherent x-rays generated using short, optical pulses.
In one aspect, the invention provides a method and apparatus for measuring a property (e.g., thickness or composition) of at least one film in a sample (e.g., a multilayer film stack contained in a microelectronic device). This embodiment includes the steps of: i) generating an optical pulse; ii) focussing the optical pulse onto or in a material to generate a coherent x-ray pulse; iii) delivering the coherent x-ray pulse to a region on the sample to generate a signal beam; iv) detecting the signal beam to generate an electrical signal; and v) analyzing the electrical signal to determine the property (e.g., thickness) of the film.
In some cases, the coherent x-ray pulse could be generated directly from a laser source, without the need for steps (i) and (ii).
In a preferred embodiment, the optical pulse has duration of 5 picoseconds or less, and is generated with a laser that includes titanium:sapphire as a lasing medium. To generate the coherent x-ray beam, the optical pulse can be focused into a gas-containing container or into an optical fiber that includes at gas-containing region. The gas used in the fiber or container typically includes argon, helium, hydrogen, or nitrogen. X-ray pulses generated by this method typically have a duration that is less than twice the duration of the optical pulse, can be focused to extremely small spot sizes (e.g., on the order of a few microns or less), and have very low spatial divergence.
In one embodiment, the coherent x-ray pulse initiates fluorescence (e.g., optical or x-ray fluorescence) in the sample that forms the signal. In this case, the signal is not necessarily coherent, e.g., it can be incoherent fluorescence. The x-ray-induced fluorescence reaches a detector and generates an electrical signal having properties determined by the fluorescence intensity and energy that can be determined by analyzing the signal beam. In one case, the fluorescence intensity is compared to a mathematical model (e.g., a look-up table or function) that relates fluorescence intensity to the film""s thickness. In addition to intensity, the fluorescence photon energy can be compared to a similar mathematical model that relates fluorescence energy to the film""s composition.
In another embodiment, one or more x-ray beams are delivered to the sample to generate an acoustic response, such as an acoustic wave or pulse. For example, two x-ray beams can be spatially overlapped on the sample surface to form an interference pattern. The sample absorbs the interference pattern to launch acoustic waves that propagate in the plane of the film. Then an optical or x-ray probe beam irradiates the acoustic waves to generate the signal beam. In one case, the acoustic waves diffract a portion of the probe beam to form a signal beam modulated at a frequency that matches the frequency of the acoustic waves. The detecting step then further includes measuring and analyzing the diffracted portion of the probe beam to determine its frequency. The frequency can then be analyzed with a mathematical model to determine a thickness of the film.
Alternatively, a single x-ray beam can irradiate the sample""s surface to initiate an acoustic wave or pulse that propagates perpendicular to the plane of the film. Here, a portion of the probe beam reflects off the acoustic response, and is detected and analyzed to determine a time-dependent feature (e.g., a series of echoes spaced in time). In this case, the time-dependent feature is analyzed to determine a thickness of the film.
The present invention has many advantages. In general, the invention features a non-contact measurement technique that simultaneously and effectively measures the thickness and composition of multiple thin films contained in a multi-layer structure. These properties can then be used to control a fabrication process (e.g., fabrication of a microelectronic device).
The invention features all the advantages of x-ray metrology, for example, each measurement is non-contact and remote (in the present invention, the measurement system can be as far as 10 cm or more from the sample). In addition, the invention can also make measurements over very small (sub-micron) regions, which can not be performed by known x-ray metrology. More particularly, this feature permits measurement of very small test areas (typically 100xc3x97100 microns) and possibly even actual features (typically 0.2 microns or less) contained in microelectronic devices. Other properties besides film thickness (e.g. composition) may also be measured more precisely through the use of the invention.
Other advantages include short pulse duration, which enables measurement of time-dependent dynamics of a sample. For example, x-ray-based measurement such as diffraction can be performed with a time resolution on the order of the pulse duration. In addition, the high peak intensity of the coherent x-rays enables measurements based on nonlinear responses of the sample (e.g. two-photon absorption and resultant fluorescence or other responses, harmonic generation, transient changes in absorption or refractive index in either the x-ray or other spectral region). The high spatial coherence enables small, micron-sized measurement area and additionally enables interference-based measurements, such as four-wave mixing, and facilitates alignment and measurement of a reflected beam or other coherent signals.
Other features, aspects, and advantages of the invention follow from the following detailed description, and from the claims.